1. Field of the Invention
This invention relates, in general, to input/output processing, and in particular, to processing data associated with or generated by input/output processing.
2. Description of Background
Input/output (I/O) operations are used to transfer data between memory and I/O devices of an I/O processing system. Specifically, data is written from memory to one or more I/O devices, and data is read from one or more I/O devices to memory by executing I/O operations.
To facilitate processing of I/O operations, an I/O subsystem of the I/O processing system is employed. The I/O subsystem is coupled to main memory and the I/O devices of the I/O processing system and directs the flow of information between memory and the I/O devices. One example of an I/O subsystem is a channel subsystem. The channel subsystem uses channel paths as communications media. Each channel path includes a channel coupled to a control unit, the control unit being further coupled to one or more I/O devices.
The channel subsystem may employ channel command words (CCWs) to transfer data between the I/O devices and memory. A CCW specifies the command to be executed. For commands initiating certain I/O operations, the CCW designates the memory area associated with the operation, the action to be taken whenever a transfer to or from the area is completed, and other options.
During I/O processing, a list of CCWs is fetched from memory by a channel. The channel parses each command from the list of CCWs and forwards a number of the commands, each command in it's own entity, to a control unit coupled to the channel. The control unit then processes the commands. The channel tracks the state of each command and controls when the next set of commands are to be sent to the control unit for processing. The channel ensures that each command is sent to the control unit in it's own entity. Further, the channel infers certain information associated with processing the response from the control unit for each command.
The control unit may also send various messages to the channel, such as acknowledgement messages indicating whether a command has been executed, and status messages providing information regarding the status of an I/O device.
The channel may send commands to the control unit in the form of Information Units (IUs). Various exchanges and sequences occur between the control unit and the channel. For example, the channel may open an exchange with the control unit and send various commands (command data) to the control unit. In response, in addition to performing other functions, the control unit may respond to the channel on the same exchange, or open a new exchange and send a response thereon.
In addition, the control unit may send unsolicited data to the channel, such as a message indicating the status of an I/O device, to which the channel may respond accordingly, such as with an acknowledgement (“ACK”) message.
A condition may arise where a message sent from the channel arrives at the control unit at the same time that a message sent from the control unit arrives at the channel, or the messages otherwise cross paths in a logical path. In this condition, both the channel and control unit receive an unsolicited message in the absence of, or prior to, receiving a response to their respective messages. This condition may be referred to as a “frames passing” or “ships passing” situation.
For ships passing conditions occurring in channel subsysytems using a common link protocol, such as the Fibre Connectivity (FICON) protocol, the inbound (toward the channel) and outbound (away from the channel) exchanges are used to resolve the condition. However, in situations where the channel and control unit use different protocols, there is no way to efficiently discontinue an operation.
Ships passing rules utilized in one protocol or mode may not work with other protocols or modes. For example, a first protocol, such as FICON, may not work with a second link protocol. The second link protocol may not be amiable to the FICON behavior, and thus there may be no way for the channel to efficiently discontinue the second link protocol operation.
Accordingly, there is a need in the art for a method and system for resolving ships passing conditions in channel communications. There is also a need in the art for resolving ships passing conditions among disparate link protocols.